(1) Field of the Invention
The present invention relates to electrode plates for batteries and methods for fabricating the same, and more particularly relates to electrode plates for batteries whose longitudinal surfaces and lateral end surface are covered with a continuous porous film and methods for fabricating the same.
(2) Description of Related Art
Primary batteries, such as dry batteries and lithium primary batteries, and secondary batteries, such as lead acid batteries, alkali storage batteries, and lithium secondary batteries, are now widely used for various applications including cell phones, notebook computers, digital cameras, electric power tools, and further hybrid vehicles. Under such circumstances, there have been demands for further increases in the capacity and power of such batteries.
Meanwhile, if an internal short circuit between a positive electrode and a negative electrode occurred inside a battery due to internal factors, such as the mixing of a foreign substance into the battery, or external factors, such as the generation of an external pressure, the increases in the capacity and power of the battery would cause a large current to flow into the short-circuited part of the battery, leading to heat generation of the battery or other problems.
In order to prevent the occurrence of the internal short circuit as described above, the following battery has been suggested (for example, Patent Document 1: Japanese Unexamined Patent Application Publication No. 7-220759). For a battery of a known example, a porous film containing a resin binder and solid particles, such as alumina powder, is formed to cover the longitudinal surfaces of any one of a positive electrode and a negative electrode. Methods for applying a coating fluid that will become a porous film to the longitudinal surfaces of an electrode plate include, for example, a gravure printing method (For example, Patent Document 2: Japanese Unexamined Patent Application Publication No. 9-298058).
By the way, in order to prevent an internal short circuit from being caused due to the separation of a porous film from an electrode plate or a crack in the porous film, a technique has been disclosed in which insulators are formed on the lateral end surfaces of an electrode plate group (for example, Patent Document 3: Japanese Unexamined Patent Application Publication No. 2005-190912). The insulators are formed in the following manner: After the preparation of the electrode plate group, the lateral end surfaces of the electrode plate group are soaked in a heated solution obtained by mixing 1000 grams of insulative particles made of alumina and 1000 grams of hot melt adhesive together, and then the soaked lateral end surfaces of the electrode plate group are cooled.